Prior Art
Prior Art Track Writers (STW) use a clock head which is captive to the STW and which is temporarily brought adjacent to one of the disc surfaces during track writing. This requirement compromises the mechanical integrity of the head disc assembly (HDA) since provision must be made in the HDA design to allow access for the STW Clock Head.
A clock track is written with the clock head using a trial and error process. The clock track is preferably "splice free". Usually a tolerance of some +/-30 nanoseconds discontinuity is allowed at the splice point. Since spindle motor speed variations are usually +/-0.05% and the nominal revolution period is 16.67 milliseconds, the period of one revolution varies by about +/-8 microseconds. An index available from the motor or from a "scratch" transition pair written with the head is used to drive a phase lock loop (PLL) which multiplies this approximately 60 Hz clock to a much higher frequency. A successful clock is written only when the correct number of the said high frequency clocks has elapsed between successive indices and the time discrepancy of the last clock time is less than 30 nanoseconds. This process takes up to a minute because of the large discrepancy between the splice error permitted and the spindle speed tolerance. The invention avoids the use of a clock head and also avoids the need for a clock track. The clock track is replaced by a "Master Track" which is written in about 20 revolutions or 0.33 seconds.
A description of a prior art clock writing procedure is available in U.S. Pat. No. 3,540,022 issued to James K. Berger. Another prior art system, U.S. Pat. No. 4,131,920, also issued to James K. Berger, discloses an improved method of clock writing. However, this still relies on trial and error for splice free tracks and, although clock writing is accomplished with a sealed HDA, it has the serious disadvantage that an expensive clock head must be included with each disc drive.